1. Field of the Invention
The present invention relates to a Pirani vacuum gauge which is widely used for pressure measurement in a low to high vacuum range by use of heat conduction of a gas.
2. Related Background Art
As disclosed in Japanese Patent No. 3188752, in conventional Pirani vacuum gauges, a heat filament made of metal wire is suspended and heated in a vacuum. When gas molecules having a lower temperature than that of the heat filament which is in a high-temperature state collide with the heat filament, the colliding gas molecules conduct heat away from the heat filament. The temperature of the heat filament thereby changes. A temperature change corresponding to the amount of heat conducted away is electrically converted and detected as a change in electrical resistance of the heat filament, and the change in electrical resistance is further converted to a pressure value, to thereby determine a gas pressure. In most general Pirani vacuum gauges presently available on the market, applied power is automatically controlled by a control circuit such that the temperature of the heat filament is always constant. This type of Pirani vacuum gauge is called a constant temperature Pirani vacuum gauge, in which the amount of heat lost is measured by constantly compensating for the amount of heat conducted away from the heat filament by the gas molecules. In this case, the power to be applied such that the temperature of the heat filament is constant is measured and converted to the gas pressure.
In the conventional vacuum gauges, there is such a problem that measured pressure values are widely varied depending on a mounting posture, that is, depending on whether the filament is vertical or horizontal in a gas range from about 104 Pa to atmosphere pressure. In order to solve the problem, a vacuum gauge which reduces variation in measured pressure values due to the posture difference by covering a part over 80% of the length of a filament with a pipe has been proposed (Pamphlet of International Publication No. 2006/057148).
However, the Pirani vacuum gauge disclosed in Pamphlet of International Publication No. 2006/057148 also has problems as described below.
One of the problems is that it takes several tens of seconds to several minutes to indicate a normal pressure value in a case where a gas pressure rapidly rises from a medium vacuum degree or less to a low vacuum degree by, for example, gas introduction into a chamber to be measured for pressure. Here, the medium vacuum degree is a pressure of about 10 Pa, and the low vacuum degree is a pressure of 1000 Pa or more. A pressure reading temporarily shows a pressure value higher than the normal pressure value until the Pirani vacuum gauge indicates the normal pressure value. When the gas pressure rapidly rises, gas molecules rapidly flow to not only collide with the heat filament, but also collide with the inside wall surface or the like of a container in which the heat filament is accommodated (hereinafter referred to as “container”). Thus, the gas molecules also conduct heat away from the inside wall surface or the like of the container, and the temperature of the container rapidly drops.
Generally, an amount of heat Q conducted away from a heat filament 7 is expressed by the next equation.Q=Kc·P·(Tf−Tw)+KR·(Tf4−Tw4)+Heat loss at end  (1)
Kc: Heat conduction coefficient of the amount of heat transferred by a gas
P: Gas pressure
Tf: Filament temperature
Tw: Temperature of the wall surface around the filament
KR: Heat radiation coefficient
The first term of the above equation (1) represents heat conduction loss that is conducted away from the heat filament by gas molecules in a container. Here, the heat conduction coefficient Kc of the amount of heat transferred by a gas is a constant number. The filament temperature Tf is controlled to be constant by a control circuit. The temperature Tw of the wall surface around the filament is determined by the temperature of surroundings where the Pirani vacuum gauge is installed.
The second term of the above equation represents loss by heat radiation from the heat filament 7 to the inside wall surface of the container. The third term represents heat conduction loss to the outside through a member for supporting the heat filament 7, to which the heat filament 7 is connected, and a lead wire connected thereto.
That is, according to the above equation (1), when the temperature Tw of the wall surface around the heat filament drops, the amount of heat Q conducted away from the heat filament is increased and the gas pressure is also indicated to be higher in proportion to the amount of heat Q. However, the temperature drop of the container is only temporary, and the temperature reaches the same temperature as that in the container in several tens of seconds to several minutes after the rapid flow of gas molecules recedes.
It is an object of the present invention to solve the aforementioned problems. That is, an object of the present invention is to provide a Pirani vacuum gauge in which restrictions on a mounting direction on a chamber to be measured for pressure are eliminated and a response to a rapid pressure rise is improved.
Another object of the present invention is to provide a Pirani vacuum gauge whose usability is improved.